It grates when the same relatively-uncommon words appear too close together in an essay or other piece of prose. One suspects that the author doesn't know enough about the topic to use precise terminology ... or can't perceive fine distinctions and nuances ... or, worst of all, just doesn't care enough to write well. If every mathematical equation is merely a "formula", or every color is a monosyllabic grunt, or every species of creature is a nondescript "animal", how much can a reader trust the accuracy of the discussion? And in contrast, if a commentary begins with "linear differential operators" and proceeds to focus on "Laplace's equation" and "second-order numerical integration methods" then even if one doesn't know the field in any detail, one can at least hope to see something with a bit of depth. Likewise when hues include mauve, taupe, and chartreuse, or when a kinkajou is also Cercoleptes caudivolvulus, aka honey bear. (Disclaimer: I have no idea what the previous sentence means!)

"Zipf's Law" postulates that the Nth most common word in a large body of text occurs roughly 1/N as often as the commonest one. Following that principle, I like to assign a tongue-in-cheek value to each word based on its general frequency of usage. "THE", "AND", "OF", "TO", "IN", "THAT", and their ilk in the top ten are dirt-cheap, less than a thousandth of a cent apiece. The top hundred words cost under a hundredth of a cent each. And so forth, with the Nth most often seen linguistic unit priced at N/100,000 dollars per use.

But if a $10 word like "sesquipedalian" gets used too much then it quickly becomes devalued ... and "too often" varies inversely with the price of the word in question. Let the writer beware!

A singular exception to this guideline was pointed out many years ago by Kip Thorne, my thesis advisor. Kip insisted that when his students wrote for publication their prose had to be well crafted, with a rich vocabulary. But clarity was even more crucial. If a particular idea was of central importance, Kip's advice was to establish a unique word or phrase for it and repeatedly use that same terminology. As he put it, establish a resonance in the reader's mind and then make it ring like a bell whenever the concept arises.Bong!